Glyceraldehyde-3-phosphate dehydrogenase is a major protein associated with the plasma membrane of retinal photoreceptor outer segments

Human cone elongation responses can be explained by photoactivated cone opsin and membrane swelling and osmotic response to phosphate produced by RGS9-catalyzed GTPase

Optical coherence tomography has established that human cone photoreceptor outer segments elongate in response to stimuli bleaching large fractions of their visual pigment. Elongation responses are completely described over their 200-fold bleaching range as the sum of two exponentially rising components differing 13-fold in time constants and 4-fold in light sensitivity. Bleaching measurements of individual cones with adaptive optics scanning laser ophthalmoscopy (SLO) suggest that component 2 arises from cone opsin and disk membrane swelling triggered by photoactivation. Application of a model of phototransduction suggests that component 1 corresponds to free phosphate generated by regulator of G-protein signaling 9 (RGS9)-catalyzed hydrolysis of guanosine triphosphate (GTP) in the α-subunit of G protein complexed with phosphodiesterase.

Human cone outer segment (COS) length changes in response to stimuli bleaching up to 99% of L- and M-cone opsins were measured with high resolution, phase-resolved optical coherence tomography (OCT). Responses comprised a fast phase (∼5 ms), during which COSs shrink, and two slower phases (1.5 s), during which COSs elongate. The slower components saturated in amplitude (∼425 nm) and initial rate (∼3 nm ms⁻¹) and are well described over the 200-fold bleaching range as the sum of two exponentially rising functions with time constants of 80 to 90 ms (component 1) and 1,000 to 1,250 ms (component 2). Measurements with adaptive optics reflection densitometry revealed component 2 to be linearly related to cone pigment bleaching, and the hypothesis is proposed that it arises from cone opsin and disk membrane swelling triggered by isomerization and rate-limited by chromophore hydrolysis and its reduction to membrane-localized all-trans retinol. The light sensitivity and kinetics of component 1 suggested that the underlying mechanism is an osmotic response to an amplified soluble by-product of phototransduction. The hypotheses that component 1 corresponds to G-protein subunits dissociating from the membrane, metabolites of cyclic guanosine monophosphate (cGMP) hydrolysis, or by-products of activated guanylate cyclase are rejected, while the hypothesis that it corresponds to phosphate produced by regulator of G-protein signaling 9 (RGS9)-catalyzed hydrolysis of guanosine triphosphate (GTP) in G protein–phosphodiesterase complexes was found to be consistent with the results. These results provide a basis for the assessment with optoretinography of phototransduction in individual cone photoreceptors in health and during disease progression and therapeutic interventions.

Energy sources that fuel metabolic processes in protruding finger-like organelles

Cells possess a variety of organelles with characteristic structure and subcellular localization intimately linked to their specific function. While most are intracellular and found in virtually all eukaryotic cells, there is a small group of organelles of elongated cylindrical shapes in highly specialized cells that protrude into the extracellular space, such as cilia, flagella, and microvilli. The ATP required by intracellular organelles is amply available in the cytosol, largely generated by mitochondria. However, such is not the case for cilia and flagella, whose slender structures cannot accommodate mitochondria. These organelles consume massive amounts of ATP to carry out high energy-demanding functions, such as sensory transduction or motility. ATP from the nearest mitochondria or other reactions within the cell body is severely limited by diffusion and generally insufficient to fuel the entire length of cilia and flagella. These organelles overcome this fuel restriction by local generation of ATP, using mechanisms that vary depending on the nutrients that are available in their particular external environment. Here, we review, with emphasis in mammals, the remarkable adaptations that cilia and flagella use to fuel their metabolic needs. Additionally, we discuss how a decrease in nutrients surrounding olfactory cilia might impair olfaction in COVID-19 patients.

Occurrence of major anti-retinal autoantibodies associated with paraneoplastic autoimmune retinopathy

Autoantibodies (AAbs) against retinal antigens can be found in patients with cancer and unexplained vision loss unrelated to the cancer metastasis. Cancer-associated retinopathy (CAR) is a rare paraneoplastic visual syndrome mediated by AAbs. Our goal was to determine whether CAR patients with different malignancies have a specific AAb or repertoire of AAbs that could serve as biomarkers for retinal disease. We found AAbs against 12 confirmed retinal antigens, with α-enolase being the most frequently recognized. The significant finding of the study was a high incidence of anti-aldolase AAbs in colon-CAR, anti-CAII in prostate-CAR, and anti-arrestin in skin melanoma patients thus these AAbs could serve as biomarkers in the context of clinical presentation and could support the diagnosis of CAR. However, a lack of AAb restriction to any one antigenic protein or to one retinal cellular location makes screening for a CAR biomarker challenging.

Glucose metabolism in mammalian photoreceptor inner and outer segments

Photoreceptors are the first-order neurons of the visual pathway, converting light into electrical signals. Rods and cones are the two main types of photoreceptors in the mammalian retina. Rods are specialized for sensitivity at the expense of resolution and are responsible for vision in dimly lit conditions. Cones are responsible for high acuity central vision and colour vision. Many human retinal diseases are characterized by a progressive loss of photoreceptors. Photoreceptors consist of four primary regions: outer segments, inner segments, cell bodies and synaptic terminals. Photoreceptors consume large amounts of energy, and therefore, energy metabolism may be a critical juncture that links photoreceptor function and survival. Cones require more energy than rods, and cone degeneration is the main cause of clinically significant vision loss in retinal diseases. Photoreceptor segments are capable of utilizing various energy substrates, including glucose, to meet their large energy demands. The pathways by which photoreceptor segments meet their energy demands remain incompletely understood. Improvements in the understanding of glucose metabolism in photoreceptor segments may provide insight into the reasons why photoreceptors degenerate due to energy failure. This may, in turn, assist in developing bio-energetic therapies aimed at protecting photoreceptors.

Impact of Autoantibodies against Glycolytic Enzymes on Pathogenicity of Autoimmune Retinopathy and Other Autoimmune Disorders

Autoantibodies (AAbs) against glycolytic enzymes: aldolase, α-enolase, glyceraldehyde-3-phosphate dehydrogenase, and pyruvate kinase are prevalent in sera of patients with blinding retinal diseases, such as paraneoplastic [cancer-associated retinopathy (CAR)] and non-paraneoplastic autoimmune retinopathies, as well as in many other autoimmune diseases. CAR is a degenerative disease of the retina characterized by sudden vision loss in patients with cancer and serum anti-retinal AAbs. In this review, we discuss the widespread serum presence of anti-glycolytic enzyme AAbs and their significance in autoimmune diseases. There are multiple mechanisms responsible for antibody generation, including the innate anti-microbial response, anti-tumor response, or autoimmune response against released self-antigens from damaged, inflamed tissue. AAbs against enolase, GADPH, and aldolase exist in a single patient in elevated titers, suggesting their participation in pathogenicity. The lack of restriction of AAbs to one disease may be related to an increased expression of glycolytic enzymes in various metabolically active tissues that triggers an autoimmune response and generation of AAbs with the same specificity in several chronic and autoimmune conditions. In CAR, the importance of serum anti-glycolytic enzyme AAbs had been previously dismissed, but the retina may be without pathological consequence until a failure of the blood–retinal barrier function, which would then allow pathogenic AAbs access to their retinal targets, ultimately leading to damaging effects.

Light-induced disulfide dimerization of recoverin under ex vivo and in vivo conditions

Despite vast knowledge of the molecular mechanisms underlying photochemical damage of photoreceptors, linked to progression of age-related macular degeneration, information on specific protein targets of the light-induced oxidative stress is scarce. Here, we demonstrate that prolonged intense illumination (halogen bulb, 1500 lx, 1-5 h) of mammalian eyes under ex vivo (cow) or in vivo (rabbit) conditions induces disulfide dimerization of recoverin, a Ca(2+)-dependent inhibitor of rhodopsin kinase. Western blotting and mass spectrometry analysis of retinal extracts reveals illumination time-dependent accumulation of disulfide homodimers of recoverin and its higher order disulfide cross-linked species, including a minor fraction of mixed disulfides with intracellular proteins (tubulins, etc.). Meanwhile, monomeric bovine recoverin remains mostly reduced. These effects are accompanied by accumulation of disulfide homodimers of visual arrestin. Histological studies demonstrate that the light-induced oxidation of recoverin and arrestin occurs in intact retina (illumination for 2 h), while illumination for 5 h is associated with damage of the photoreceptor layer. A comparison of ex vivo levels of disulfide homodimers of bovine recoverin with redox dependence of its in vitro thiol-disulfide equilibrium (glutathione redox pair) gives the lowest estimate of redox potential in rod outer segments under illumination from -160 to -155 mV. Chemical crosslinking and dynamic light scattering data demonstrate an increased propensity of disulfide dimer of bovine recoverin to multimerization/aggregation. Overall, the oxidative stress caused by the prolonged intense illumination of retina might affect rhodopsin desensitization via concerted disulfide dimerization of recoverin and arrestin. The developed herein models of eye illumination are useful for studies of the light-induced thiol oxidation of visual proteins.

High-resolution crystal structures of the photoreceptor glyceraldehyde 3-phosphate dehydrogenase (GAPDH) with three and four-bound NAD molecules

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) catalyzes the oxidative phosphorylation of d-glyceraldehyde 3-phosphate (G3P) into 1,3-diphosphoglycerate (BGP) in the presence of the NAD cofactor. GAPDH is an important drug target because of its central role in glycolysis, and nonglycolytic processes such as nuclear RNA transport, DNA replication/repair, membrane fusion and cellular apoptosis. Recent studies found that GAPDH participates in the development of diabetic retinopathy and its progression after the cessation of hyperglycemia. Here, we report two structures for native bovine photoreceptor GAPDH as a homotetramer with differing occupancy by NAD, bGAPDH(NAD)4 , and bGAPDH(NAD)3 . The bGAPDH(NAD)4 was solved at 1.52 Å, the highest resolution for GAPDH. Structural comparison of the bGAPDH(NAD)4 and bGAPDH(NAD)3 models revealed novel details of conformational changes induced by cofactor binding, including a loop region (residues 54-56). Structure analysis of bGAPDH confirmed the importance of Phe34 in NAD binding, and demonstrated that Phe34 was stabilized in the presence of NAD but displayed greater mobility in its absence. The oxidative state of the active site Cys149 residue is regulated by NAD binding, because this residue was found oxidized in the absence of dinucleotide. The distance between Cys149 and His176 decreased upon NAD binding and Cys149 remained in a reduced state when NAD was bound. These findings provide an important structural step for understanding the mechanism of GAPDH activity in vision and its pathological role in retinopathies.

Yeast Tdh3 (glyceraldehyde 3-phosphate dehydrogenase) is a Sir2-interacting factor that regulates transcriptional silencing and rDNA recombination

Sir2 is an NAD(+)-dependent histone deacetylase required to mediate transcriptional silencing and suppress rDNA recombination in budding yeast. We previously identified Tdh3, a glyceraldehyde 3-phosphate dehydrogenase (GAPDH), as a high expression suppressor of the lethality caused by Sir2 overexpression in yeast cells. Here we show that Tdh3 interacts with Sir2, localizes to silent chromatin in a Sir2-dependent manner, and promotes normal silencing at the telomere and rDNA. Characterization of specific TDH3 alleles suggests that Tdh3's influence on silencing requires nuclear localization but does not correlate with its catalytic activity. Interestingly, a genetic assay suggests that Tdh3, an NAD(+)-binding protein, influences nuclear NAD(+) levels; we speculate that Tdh3 links nuclear Sir2 with NAD(+) from the cytoplasm.

A reduced zinc diet or zinc transporter 3 knockout attenuate light induced zinc accumulation and retinal degeneration

Our previous study on retinal light exposure suggests the involvement of zinc (Zn(2+)) toxicity in the death of RPE and photoreceptors (LD) which could be attenuated by pyruvate and nicotinamide, perhaps through restoration of NAD(+) levels. In the present study, we examined Zn(2+) toxicity, and the effects of NAD(+) restoration in primary retinal cultures. We then reduced Zn(2+) levels in rodents by reducing Zn(2+) levels in the diet, or by genetics and measured LD. Sprague Dawley albino rats were fed 2, or 61 mg Zn(2+)/kg of diet for 3 weeks, and exposed to 18 kLux of white light for 4 h. We light exposed (70 kLux of white light for 50 h) Zn(2+) transporter 3 knockout (ZnT3-KO, no synaptic Zn(2+)), or RPE65 knockout mice (RPE65-KO, lack rhodopsin cycling), or C57/BI6/J controls and determined light damage and Zn(2+) staining. Retinal Zn(2+) staining was examined at 1 h and 4 h after light exposure. Retinas were examined after 7 d by optical coherence tomography and histology. After LD, rats fed the reduced Zn(2+) diet showed less photoreceptor Zn(2+) staining and degeneration compared to a normal Zn(2+) diet. Similarly, ZnT3-KO and RPE65-KO mice showed less Zn(2+) staining, NAD(+) loss, and RPE or photoreceptor death than C57/BI6/J control mice. Dietary or ZnT3-dependent Zn(2+) stores, and intracellular Zn(2+) release from rhodopsin recycling are suggested to be involved in light-induced retinal degeneration. These results implicate novel rhodopsin-mediated mechanisms and therapeutic targets for LD. Our companion manuscript demonstrates that pharmacologic, circadian, or genetic manipulations which maintain NAD(+) levels reduce LD.

NAD(+) maintenance attenuates light induced photoreceptor degeneration

Light-induced retinal damage (LD) occurs after surgery or sun exposure. We previously showed that zinc (Zn(2+)) accumulated in photoreceptors and RPE cells after LD but prior to cell death, and pyruvate or nicotinamide attenuated the resultant death perhaps by restoring nicotinamide adenine dinucleotide (NAD(+)) levels. We first examined the levels of NAD(+) and the efficacy of pyruvate or nicotinamide in oxidative toxicities using primary retinal cultures. We next manipulated NAD(+) levels in vivo and tested the affect on LD to photoreceptors and RPE. NAD(+) levels cycle with a 24-h rhythm in mammals, which is affected by the feeding schedule. Therefore, we tested the affect of increasing NAD(+) levels on LD by giving nicotinamide, inverting the feeding schedule, or using transgenic mice which overexpress cytoplasmic nicotinamide mononucleotide adenyl-transferase-1 (cytNMNAT1), an NAD(+) synthetic enzyme. Zn(2+) accumulation was also assessed in culture and in retinal sections. Retinas of light damaged animals were examined by OCT and plastic sectioning, and retinal NAD(+) levels were measured. Day fed, or nicotinamide treated rats showed less NAD(+) loss, and LD compared to night fed rats or untreated rats without changing the Zn(2+) staining pattern. CytNMNAT1 showed less Zn(2+) staining, NAD(+) loss, and cell death after LD. In conclusion, intense light, Zn(2+) and oxidative toxicities caused an increase in Zn(2+), NAD(+) loss, and cell death which were attenuated by NAD(+) restoration. Therefore, NAD(+) levels play a protective role in LD-induced death of photoreceptors and RPE cells.

Diversity in autoimmunity against retinal, neuronal, and axonal antigens in acquired neuro-retinopathy

PURPOSE

Autoimmune retinopathies and optic neuropathies are complex disorders of the retina and the optic nerve, in which patients develop autoantibodies (AAbs) against retinal and optic nerve proteins. Autoimmunity might significantly influence the outcome of retinal and optic nerve degenerative process but the pathogenic process is not fully elucidated. To better understand the role of AAbs in pathogenicity of these suspected autoimmune visual disorders, we focused on unique AAbs specificities associated with the syndrome to identify their antigenic targets in the optic nerve and retina.

METHODS

Serum samples were obtained from patients, whose visual disorders were potentially autoimmune in nature, including patients with cancer with possible paraneoplastic syndrome. Autoantibodies were tested against human optic nerve and retinal antigens for specificity by Western blotting and immunofluorescence.

RESULTS

Out of 209 tested for anti-optic nerve autoantibodies, 55% showed specific neuronal autoantibodies. The repertoire of anti-optic nerve autoantibodies often differed from anti-retinal antibodies. The major antigenic targets for these antibodies could be divided into four groups. Autoantibodies specific to classical glycolytic enzymes involved in energy production (α and γ enolases, glyceraldehyde 3-phosphate dehydrogenase) also reacted with retinal antigens. Autoantibodies targeted neuronal-specific myelin proteins (MBP, MOG), aquaporin 4, and collapsing response mediator protein 5 reacted with optic nerve antigens. They showed immunostaining of axons and myelin in the optic nerve as determined by double immunofluorescence.

CONCLUSION

We identified novel neuronal autoantigens not previously known to be associated with acquired autoimmune retinopathy and optic neuropathy. Knowledge of the full autoantibody repertoire perpetuating this syndrome is an important first requirement in increasing our understanding of the autoimmune process to facilitate better diagnosis, prognosis, and treatment.

Extramitochondrial tricarboxylic acid cycle in retinal rod outer segments

Vertebrate retinal rod Outer Segments (OS) are the site of visual transduction, an energy demanding process for which mechanisms of ATP supply are still poorly known. Glycolysis or diffusion of either ATP or phosphocreatine from the Inner Segment (IS) does not seem to display adequate timing to supply ATP for phototransduction. We have previously reported data suggesting an aerobic metabolism in OS, which would largely account for the light-stimulated ATP need of the photoreceptor. Here, by oxymetry and biochemical analyses we show that: (i) disks isolated by Ficoll flotation consume O(2) in the presence of physiological respiring substrates either in coupled or uncoupled conditions; (ii) OS homogenates contain the whole biochemical machinery for the degradation of glucose, i.e. glycolysis and the tricarboxylic acid cycle (TCA cycle), consistently with the results of our previous proteomic study. Activities of the 8 TCA cycle enzymes in OS were comparable to those in retinal mitochondria-enriched fractions. Disk and OS preparations were subjected to TEM analysis, and while they can be considered free of inner segment contaminants, immunogold with specific antibodies demonstrate the expression therein of both the visual pigment rhodopsin and F(o)F(1)-ATP synthase. Finally, double immunofluorescence on mouse retina sections demonstrated a colocalization of some respiratory complex mitochondrial proteins with rhodopsin in rod OS. Data, suggestive of the exportability of the mitochondrial machinery for aerobic metabolism, may shed light on those retinal pathologies related to energy supply impairment in OS and to mutations in TCA enzymes.

Proteomic analysis of the retinal rod outer segment disks

The initial events of vision at low light take place in vertebrate retinal rods. The rod outer segment consists of a stack of flattened disks surrounded by the plasma membrane. A list of the proteins that reside in disks has not been achieved yet. We present the first comprehensive proteomic analysis of purified rod disks, obtained by combining the results of two-dimensional gel electrophoresis separation of disk proteins to MALDI-TOF or nLC-ESI-MS/MS mass spectrometry techniques. Intact disks were isolated from bovine retinal rod outer segments by a method that minimizes contamination from inner segment. Out of a total of 187 excised spots, 148 proteins were unambiguously identified. An additional set of 61 proteins (partially overlapping with the previous ones) was generated by one-dimensional (1D) gel nLC-ESI-MS/MS method. Proteins involved in vision as well as in aerobic metabolism were found, among which are the five complexes of oxidative phosphorylation. Results from biochemical, Western blot, and confocal laser scanning microscopy immunochemistry experiments suggest that F 1F o-ATP synthase is located and catalytically active in ROS disk membranes. This study represents a step toward a global physiological characterization of the disk proteome and provides information necessary for future studies on energy supply for phototransduction.

Light-dependent compartmentalization of transducin in rod photoreceptors

Three major visual signaling proteins, transducin, arrestin, and recoverin undergo bidirectional translocations between the outer segment and inner compartments of rod photoreceptors in a light-dependent manner. The light-dependent translocation of proteins is believed to contribute to adaptation and neuroprotection of photoreceptor cells. The potential physiological significance and mechanisms of light-controlled protein translocations are at the center of current discussion. In this paper, I outline the latest advances in understanding the mechanisms of bidirectional translocation of transducin and determinants of its steady-state distribution in dark- and light-adapted photoreceptor cells.

Proteomics of photoreceptor outer segments identifies a subset of SNARE and Rab proteins implicated in membrane vesicle trafficking and fusion

The outer segment is a specialized compartment of vertebrate rod and cone photoreceptor cells where phototransduction takes place. In rod cells it consists of an organized stack of disks enclosed by a separate plasma membrane. Although most proteins involved in phototransduction have been identified and characterized, little is known about the proteins that are responsible for outer segment structure and renewal. In this study we used a tandem mass spectrometry-based proteomics approach to identify proteins in rod outer segment preparations as an initial step in defining their roles in photoreceptor structure, function, renewal, and degeneration. Five hundred and sixteen proteins were identified including 41 proteins that function in rod and cone phototransduction and the visual cycle and most proteins previously shown to be involved in outer segment structure and metabolic pathways. In addition, numerous proteins were detected that have not been previously reported to be present in outer segments including a subset of Rab and SNARE proteins implicated in vesicle trafficking and membrane fusion. Western blotting and immunofluorescence microscopy confirmed the presence of Rab 11b, Rab 18, Rab 1b, and Rab GDP dissociation inhibitor in outer segments. The SNARE proteins, VAMP2/3, syntaxin 3, N-ethylmaleimide-sensitive factor, and Munc 18 detected in outer segment preparations by mass spectrometry and Western blotting were also observed in outer segments by immunofluorescence microscopy. Syntaxin 3 and N-ethylmaleimide- sensitive factor had a restricted localization at the base of the outer segments, whereas VAMP2/3 and Munc 18 were distributed throughout the outer segments. These results suggest that Rab and SNARE proteins play a role in vesicle trafficking and membrane fusion as part of the outer segment renewal process. The data set generated in this study is a valuable resource for further analysis of photoreceptor outer segment structure and function.

Interaction of glyceraldehyde-3-phosphate dehydrogenase in the light-induced rod alpha-transducin translocation

The light-dependent subcellular translocation of rod alpha-transducin (GNAT-1, or rod Talpha) has been well documented. In dark-adapted animals, rod Talpha (rTalpha) is predominantly located in the rod outer segment (ROS) and translocates into the rod inner segment (RIS) upon exposure to the light. Neither the molecular participants nor the mechanism(s) involved in this protein trafficking are known. We hypothesized that other proteins must interact with rTalpha to affect the translocations. Using the MBP-rTalpha fusion pulldown assay, the yeast two-hybrid assay and the co-immunoprecipitation assay, we identified glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and rTalpha as interacting proteins. Immunoprecipitation also showed beta-actin associates with rTalpha in the dark but not in the light. To further investigate the involvement of GAPDH in light-induced rod Talpha translocation, GAPDH mRNA was knocked down in vivo by transient expression of siRNAs in rat photoreceptor cells. Under completely dark- and light-adapted conditions, the translocation of rTalpha was not significantly different within the 'GAPDH knock-down photoreceptor cells' compared to the non-transfected control cells. However, under partial dark-adaptation, rTalpha translocated more slowly in the 'GAPDH knock-down cells' supporting the conclusion that GAPDH is involved in rTalpha translocation from the RIS to the ROS during dark adaptation.

Entamoeba histolytica: ADP-ribosylation of secreted glyceraldehyde-3-phosphate dehydrogenase

In addition to its classic glycolytic role, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) has been implicated in many activities unrelated to glycolysis, such as membrane fusion, binding to host proteins and signal transduction. GAPDH can be the target of several modifications that allow incorporation to membranes and possible regulation of its activity; among these modifications is mono-ADP-ribosylation. This post-translational modification is important for the regulation of many cellular processes and is the mechanism of action of several bacterial toxins. In a previous study, we observed the extracellular ADP-ribosylation of a 37-kDa ameba protein. We report here that GAPDH and cysteine synthase A are the main ADP-ribosylated proteins in Entamoeba histolytica extracellular medium, GAPDH is secreted from ameba at 37 degrees C in a time-dependent manner, and its enzymatic activity is not inhibited by ADP-ribosylation. Extracellular GAPDH from ameba may play an important role in the survival of this human pathogen or in interaction with host molecules, as occurs in other organisms.

Hair bundles are specialized for ATP delivery via creatine kinase

When stimulated strongly, a hair cell's mechanically sensitive hair bundle may consume ATP too rapidly for replenishment by diffusion. To provide a broad view of the bundle's protein complement, including those proteins participating in energy metabolism, we used shotgun mass spectrometry methods to identify proteins of purified chicken vestibular bundles. In addition to cytoskeletal proteins, proteins involved in Ca(2+) regulation, and stress-response proteins, many of the most abundant bundle proteins that were identified by mass spectrometry were involved in ATP synthesis. After beta-actin, the cytosolic brain isoform of creatine kinase was the next most abundant bundle protein; at approximately 0.5 mM, creatine kinase is capable of maintaining high ATP levels despite 1 mM/s ATP consumption by the plasma-membrane Ca(2+)-ATPase. Consistent with this critical role in hair bundle function, the creatine kinase circuit is essential for high-sensitivity hearing as demonstrated by hearing loss in creatine kinase knockout mice.

Retinal proteins modified by 4-hydroxynonenal: identification of molecular targets

The reactive aldehyde, 4-hydroxynonenal (HNE), is a product of lipid peroxidation that can covalently modify and inactivate proteins. Previously, we reported increased HNE modification of select retinal proteins resolved by one-dimensional gel electrophoresis in aged Fisher 344 x Brown Norway rats (Louie, J.L., Kapphahn, R.J., Ferrington, D.A., 2002. Proteasome function and protein oxidation in the aged retina. Exp. Eye Res. 75, 271-284). In the current study, quantitative assessment of HNE molar content using slot blot immunoassays showed HNE content is increased 30% in aged rat retina. In contrast, there was no age-related difference in HNE content in individual spots resolved by 2D gel electrophoresis suggesting the increased modification is likely on membrane proteins that are missing on 2D gels. The HNE-immunoreactive proteins resolved by 2D gel electrophoresis were identified by MALDI-TOF mass spectrometry. These proteins are involved in metabolism, chaperone function, and fatty acid transport. Proteins that were frequently modified and had the highest molar content of HNE included triosephosphate isomerase, alpha enolase, heat shock cognate 70 and betaB2 crystallin. Immunochemical detection of HNE adducts on retinal sections showed greater immune reaction in ganglion cells, photoreceptor inner segment, and the inner plexiform layer. Identification of HNE modified proteins in two alternative model systems, human retinal pigment epithelial cells in culture (ARPE19) and human donor eyes, indicated that triosephosphate isomerase and alpha enolase are generally modified. These results identify a common subset of proteins that contain HNE adducts and suggest that select retinal proteins are molecular targets for HNE modification.

Neurospora crassa CyPBP37: a cytosolic stress protein that is able to replace yeast Thi4p function in the synthesis of vitamin B1

Recently, we identified CyPBP37 of Neurospora crassa as a binding partner of cyclophilin41. CyPBP37 function had not yet been described, although orthologs in other organisms have been implicated in the biosynthesis of the thiazole moiety of thiamine (vitamin B1) and/or stress-related pathways. Here, CyPBP37 is characterized as an abundant cytosolic protein with a functional NAD-binding site. Saccharomyces cerevisiae mutants lacking Thi4p (the CyPBP37 ortholog) are auxotrophic for vitamin B1 (thiamine) but can grow in the presence of the thiazole moiety of thiamine, suggesting a role for Thi4p in the biosynthesis of thiazole. N.crassa CyPBP37 is able to functionally replace Thi4p in yeast thiazole synthesis. Cellular fractionation studies revealed that Thi4p is a cytosolic protein in S.cerevisiae, like its ortholog CyPBP37 in N.crassa. This implies that thiamine synthesis takes place in the cytosol of both organisms and not in the mitochondria, as suggested. The expression of CyPBP37 and Thi4p is repressed by thiamine but not by thiazole in the growth medium. In addition to its function in thiazole synthesis, CyPBP37 is a stress-inducible protein. N.crassa cyclophilin41 can chaperone the folding of CyPBP37, its own binding partner.

ATP synthesis in the disk membranes of rod outer segments of bovine retina

ATP is synthesized on the disk membrane isolated from rod outer segments of the bovine retina. Together with a slow component which accounted for a constant rate of about 22 nmol ATP/min/mg of protein and which was due to the adenylate kinase activity, a fast component with a maximal activity of about 58 nmol ATP/min/mg of protein was measured at physiological calcium concentrations. This fast activity disappeared in the presence of the Ca(2+) ionophore A23187, was inhibited by vanadate or thapsigargin but not by oligomycin, suggesting that this ATP synthesis is due to the reversal functioning of the Ca(2+)-ATPase previously found on the disk membranes.

Glyceraldehyde-3-phosphate dehydrogenase is negatively regulated by ADP-ribosylation in the fungus Phycomyces blakesleeanus

Dormant spores of Phycomyces blakesleeanus contain a 37 kDa protein that is endogenously mono-ADP-ribosylated. This protein was purified and identified as glyceraldehyde-3-phosphate dehydrogenase (GAPDH) by N-terminal sequencing and homology analysis. GAPDH enzymic activity changed dramatically upon spore germination, being maximal at stages where ADP-ribosylation was nearly undetectable. The presence of glyceraldehyde 3-phosphate in this reaction affected the [(32)P]ADP-ribosylation of the GAPDH. ADP-ribosylation of the GAPDH occurred by transfer of the ADP-ribose moiety from NAD to an arginine residue. A model for the regulation of GAPDH activity and its role in spore germination in P. blakesleeanus is proposed.

Influence of exercise on the activity and the distribution between free and bound forms of glycolytic and associated enzymes in tissues of horse mackerel

The effects of short-term burst (5 min at 1.8 m/s) swimming and long-term cruiser (60 min at 1.2 m/s) swimming on maximal enzyme activities and enzyme distribution between free and bound states were assessed for nine glycolytic and associated enzymes in tissues of horse mackerel, Trachurus mediterraneus ponticus. The effects of exercise were greatest in white muscle. The activities of phosphofructokinase (PFK), pyruvate kinase (PK), fructose-1,6-bisphosphatase (FBPase), and phosphoglucomutase (PGM) all decreased to 47, 37, 37 and 67%, respectively, during 60-min exercise and all enzymes except phosphoglucoisomerase (PGI) and PGM showed a change in the extent of binding to subcellular particulate fractions during exercise. In red muscle, exercise affected the activities of PGI, FBPase, PFK, and lactate dehydrogenase (LDH) and altered percent binding of only PK and LDH. In liver, exercise increased the PK activity 2.3-fold and reduced PGI 1.7-fold only after 5 min of exercise but altered the percent binding of seven enzymes. Fewer effects were seen in brain, with changes in the activities of aldolase and PGM and in percent binding of hexokinase, PFK and PK. Changes in enzyme activities and in binding interactions with subcellular particulate matter appear to support the altered demands of tissue energy metabolism during exercise.

Adenylate kinase activity in rod outer segments of bovine retina

The rod outer segments of bovine retina contain two different adenylate kinases: a soluble activity, which is not sensitive to calcium ion, and an activity bound to disk membranes, which is dependent on the calcium levels. In fact, the maximal activity associated to the disks is reached at Ca(2+) concentrations between 10(-6) and 10(-7) M, which is the range of calcium level actually present in the rod cell. The Michaelis-Menten kinetics of the enzyme activity on disk membranes was determined and the actual concentrations of ATP, AMP and ADP were measured in the photoreceptor outer segment. Therefore, the physiological relevance of the adenylate kinase activity was discussed considering the above results. The formation of ATP catalyzed by the enzyme seems appropriate to supply at least some of the reactions necessary for phototransduction, indicating that ATP could be regenerated from ADP directly on the disk membranes where the photoreception events take place.

Identification and characterization of all-trans-retinol dehydrogenase from photoreceptor outer segments, the visual cycle enzyme that reduces all-trans-retinal to all-trans-retinol

Retinol dehydrogenase (RDH), the enzyme that catalyzes the reduction of all-trans-retinal to all-trans-retinol within the photoreceptor outer segment, was the first visual cycle enzymatic activity to be identified. Previous work has shown that this enzyme utilizes NADPH, shows a marked preference for all-trans-retinal over 11-cis-retinal, and is tightly associated with the outer segment membrane. This paper reports the identification of a novel member of the short chain dehydrogenase/reductase family, photoreceptor RDH (prRDH), using subtraction and normalization of retina cDNA, high throughput sequencing, and data base homology searches to detect retina-specific genes. Bovine and human prRDH are highly homologous and are most closely related to 17-beta-hydroxysteroid dehydrogenase 1. The enzymatic properties of recombinant bovine prRDH closely match those previously reported for RDH activity in crude bovine rod outer segment preparations. In situ hybridization and RNA blotting show that the PRRDH gene is expressed specifically in photoreceptor cells, and protein blotting and immunocytochemistry show that prRDH localizes exclusively to both rod and cone outer segments and that prRDH is tightly associated with outer segment membranes. Taken together, these data indicate that prRDH is the enzyme responsible for the reduction of all-trans-retinal to all-trans-retinol within the photoreceptor outer segment.

Progressive association of a "soluble" glycolytic enzyme with the detergent-insoluble cytoskeleton during in vitro morphogenesis of MDCK epithelial cells

In MDCK epithelial cells, cell contact at confluency initiates a protracted process of morphogenesis during which several proteins known to bind the cytoskeleton become progressively associated with the detergent-resistant cell fraction and distributed to their characteristic polarized domains. Using extraction protocols that identify this tight cytoskeletal linkage, here we show a similar but slower, time-dependent enrichment in the detergent resistant fraction of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), a highly abundant glycolytic enzyme that is traditionally considered soluble. Similar enrichment did not occur for two other glycolytic enzymes, phosphoglycerate mutase or lactate dehydrogenase. Insoluble GAPDH was not homogeneously distributed in the cytoplasm but rather displayed several discrete patterns that varied within and among MDCK cells. It also localized prominently to a few nuclei in the phenotypically heterogeneous cells of late confluency cultures. Disruptors of cytoskeletal filaments were relatively ineffective in the postconfluent epithelial monolayers, although use of disrupting agents implicated actin as the cytoplasmic filament that tethers insoluble GAPDH. Catalytic activity could be demonstrated in the insoluble fraction of GAPDH from postconfluent cultures, but only after release by mechanical disruption of insoluble extracts. Treatment of postconfluent cells with agents that deplete ATP diminished the fraction of cytoskeletally associated GAPDH, and levels of insoluble GAPDH were restored with ATP repletion, suggesting that ATP levels may regulate cytoskeletal linkage and thereby local enzyme activity. We conclude that the highly abundant and ubiquitous enzyme GAPDH becomes progressively enriched in detergent stable subcellular compartments during the process of epithelial morphogenesis. The process that produces GAPDH compartments is slow, suggesting that epithelial cells just at confluency, when they are typically analyzed, have not yet maximized the organizational state that can be attained in monolayer culture.

Rhodopsin and phototransduction

Recent studies on rhodopsin structure and function are reviewed and the properties of vertebrate as well as invertebrate rhodopsin described. Open issues such as the 'red shift' of the absorbance spectra are emphasized in the light of the present model of the retinal-binding pocket. The processes that restore the rhodopsin content in photoreceptors are also presented with a comparison between vertebrate and invertebrate visual systems. The central role of rhodopsin in the phototransduction cascade becomes evident by examining the main reports on light-activated conformational changes of rhodopsin and its interaction with transducin. Shut-off mechanisms are considered by reporting the studies on the sites of rhodopsin phosphorylation and arrestin binding. Furthermore, recent findings on the energetics of phototransduction point out that the ATP needed for photoreception in vertebrates is synthesized in the outer segments where phototransduction events take place.

Use of a proteome strategy for tagging proteins present at the plasma membrane

A plasma membrane (PM) fraction was purified from Arabidopsis thaliana using a standard procedure and analyzed by two-dimensional (2D) gel electrophoresis. The proteins were classified according to their relative abundance in PM or cell membrane supernatant fractions. Eighty-two of the 700 spots detected on the PM 2D gels were microsequenced. More than half showed sequence similarity to proteins of known function. Of these, all the spots in the PM-specific and PM-enriched fractions, together with half of the spots with similar abundance in PM fraction and supernatant, have previously been found at the PM, supporting the validity of this approach. Extrapolation from this analysis indicates that (i) approximately 550 polypeptides found at the PM could be resolved on 2D gels; (ii) that numerous proteins with multiple locations are found at the PM; and (iii) that approximately 80% of PM-specific spots correspond to proteins with unknown function. Among the later, half are represented by ESTs or cDNAs in databases. In this way, several unknown gene products were potentially localized to the PM. These data are discussed with respect to the efficiency of organelle proteome approaches to link systematically genomic data to genome expression. It is concluded that generalized proteomes can constitute a powerful resource, with future completion of Arabidopsis genome sequencing, for genome-wide exploration of plant function.

Effect of hypoxia on the activity and binding of glycolytic and associated enzymes in sea scorpion tissues

The effect of hypoxia on the levels of glycogen, glucose and lactate as well as the activities and binding of glycolytic and associated enzymes to subcellular structures was studied in brain, liver and white muscle of the teleost fish, Scorpaena porcus. Hypoxia exposure decreased glucose levels in liver from 2.53 to 1.70 mumol/g wet weight and in muscle led to its increase from 3.64 to 25.1 mumol/g wet weight. Maximal activities of several enzymes in brain were increased by hypoxia: hexokinase by 23%, phosphoglucoisomerase by 47% and phosphofructokinase (PFK) by 56%. However, activities of other enzymes in brain as well as enzymes in liver and white muscle were largely unchanged or decreased during experimental hypoxia. Glycolytic enzymes in all three tissues were partitioned between soluble and particulate-bound forms. In several cases, the percentage of bound enzymes was reduced during hypoxia; bound aldolase in brain was reduced from 36.4 to 30.3% whereas glucose-6-phosphate dehydrogenase fell from 55.7 to 28.7% bound. In muscle PFK was reduced from 57.4 to 41.7% bound. Oppositely, the proportion of bound aldolase and triosephosphate isomerase increased in hypoxic muscle. Phosphoglucomutase did not appear to occur in a bound form in liver and bound phosphoglucomutase disappeared in muscle during hypoxia exposure. Anoxia exposure also led to the disappearance of bound fructose-1,6-bisphosphatase in liver, whereas a bound fraction of this enzyme appeared in white muscle of anoxic animals. The possible function of reversible binding of glycolytic enzymes to subcellular structures as a regulatory mechanism of carbohydrate metabolism is discussed.

Inhibition of immunoglobulin production stimulating activity of glyceraldehyde-3-phosphate dehydrogenase by nucleotides

We have previously identified glyceraldehyde-3-phosphate dehydrogenase as an immunoglobulin production stimulating factor (IPSF) which facilitated immunoglobulin production by hybridomas and lymphocytes. The IPSF activity of this enzyme was suppressed by the coexistence of some sorts of nucleotides. We now report that the IPSF effect of GAPDH was suppressed by the coexistence of DNA, the inhibiting effect of degraded DNA being inferior to that of long-chain DNA. Both single-stranded and double-stranded synthetic polyribonucleotides also inhibited the IPSF activity of GAPDH. Moreover, nicotinamide adenine dinucleotide (NAD+) repressed the IPSF effect.

The mode of actions of glyceraldehyde-3-phosphate dehydrogenase identified as an immunoglobulin production stimulating factor

Immunoglobulin production stimulating factor-II alpha, which enhances immunoglobulin production of human and mouse hybridomas was purified from cell lysate of human Burkitt's lymphoma, Namalwa cells, and identified as glyceraldehyde-3-phosphate dehydrogenase. The enhancement of immunoglobulin production with this enzyme was not linked with its enzymatic activity. The enzyme enhanced immunoglobulin productivity of transcription-suppressed hybridomas, but did not enhance that of translation-suppressed hybridomas. From these results, it is suggested that this enzyme takes part in the post-translational control or the enhancement of translation activity to stimulate immunoglobulin production of hybridomas.

Characterization of rod outer segment plasma membrane proteins which bind to the mannose receptor

Previous work by our laboratory has demonstrated that rod outer segment (ROS) phagocytosis can be mediated by mannose-receptor dependent activity. This study was designed to probe for potential ligands on the ROS surface which could interact with the mannose receptor during the phagocytic cycle. Solubilized ROS plasma membranes were passed over a mannose receptor-Sepharose column in the presence of CaCl2. Proteins specifically bound to the column were eluted using methyl-D-mannoside and EDTA and characterized by gel electrophoresis, lectin blots, and immunoblots. Silver stained gels of ROS plasma membrane proteins eluted from the mannose-receptor column demonstrated six bands: a major band at 36 kD, identified by monospecific antibodies as rhodopsin, and bands of Mr = 39 kD, 67 kD, 76 kD, 97 kD and 100 kD. Lectin blots of the eluted fractions confirmed that all six proteins in these fractions could bind concanavalin A. In summary, these results showed that rhodopsin and several other mannose-containing glycoproteins on ROS plasma membranes were bound to a mannose receptor column, and thus could serve as ligands for mannose receptor-mediated ROS phagocytosis.

Characterization and localization of adenosine A2 receptors in bovine rod outer segments

Previous work from this laboratory has shown that retinal adenosine A2 binding sites are localized over outer and inner segments of photoreceptors in rabbit and mouse retinal sections. In the present study, adenosine receptor binding has been characterized and localized in membranes from bovine rod outer segments (ROS). Saturation studies with varying concentrations (10-150 nM) of 5'-(N-[2,8-3H]ethylcarboxamido)adenosine ([3H]NECA) and 100 micrograms of ROS membrane protein show a single site with a KD of 103 nM and a Bmax of 1.3 pM/mg of protein. Cold Scatchards, which used nonradiolabeled NECA (concentrations ranging from 10 nM to 250 microM) in competition with a fixed amount of [3H]NECA (30 nM), demonstrated the presence of a low-affinity site (KD, 50 microM) in addition to the high-affinity site. To confirm the presence of A2a binding sites, saturation analyses with 2-p-(2-[3H]-carboxyethyl)phenylamino-5'-N-ethylcarboxamido adenosine (0-80 nM) also revealed a single population of high-affinity A2a receptors (KD, 9.4 nM). The binding sites labeled by [3H]NECA appear to be A2 receptor sites because binding was displaced by increasing concentrations of 5'-(N-methylcarboxamido)adenosine and 2-chloroadenosine. ROS were fractionated into plasma and disk membranes for localization studies. Receptor binding assays, used to determine specific binding, showed that the greatest concentration of A2 receptors was on the plasma membranes. Therefore, adenosine A2 receptors are in a position to respond to changes in the concentration of extracellular adenosine, which may exhibit a circadian rhythm.

Brain-type creatine kinase in photoreceptor cell outer segments: role of a phosphocreatine circuit in outer segment energy metabolism and phototransduction

Different isoforms of creatine kinase, an important enzyme of vertebrate energy metabolism, were localized in bovine photoreceptor cells, with particular emphasis on the identification and quantification of the brain-type isoform within the outer segment compartment. Using immunofluorescence and immunoelectron microscopy, brain-type creatine kinase was shown to be present in bovine photoreceptor cell outer and inner segments. The presence of this isoenzyme in rod outer segments was additionally confirmed by immunoblotting and immunolabeling of isolated rod outer segments. The content of creatine kinase in rod outer segments was quantified by measuring creatine kinase activity after membrane disruption with detergent. The ATP regeneration potential provided by the creatine kinase in isolated, washed bovine rod outer segments was 1.2 +/- (0.4) i.u. mg-1 rhodopsin. This value was calculated to be at least an order of magnitude larger than that necessary to replenish the energy required for cGMP resynthesis in rod outer segments, and high enough to regenerate the entire ATP pool of rod outer segments within the time span of a photic cycle. A mitochondrial creatine kinase isoenzyme was located within the ellipsoid portions of bovine rod and cone inner segments by immunofluorescence microscopy and, using immunogold staining, was specifically localized in the mitochondria clustered within bovine rod and cone inner segments. These results suggest that vertebrate photoreceptor cells contain a functional phosphocreatine circuit. Outer segment creatine kinase may play an important role in phototransduction by providing energy for the visual cycle, maintaining high local ATP/ADP ratios and consuming protons produced by enzymes located in the outer segment.

Structural and functional properties of rhodopsin from rod outer segment disk and plasma membrane

The structural and functional properties of bovine rhodopsin from rod outer segment disk and plasma membranes were compared by high performance liquid chromatography (HPLC), mass spectrometric analyses, and in vitro rhodopsin phosphorylation assays. Disk and plasma membranes separated by a ricin gold-dextran affinity perturbation method were treated with trypsin or cyanogen bromide, and the N-terminal and C-terminal rhodopsin peptides were isolated by immunoaffinity chromatography using antirhodopsin monoclonal antibodies coupled to Sepharose. Reverse phase HPLC chromatograms of the C-terminal and N-terminal peptides from disk and plasma membrane rhodopsin were found to be similar. Mass spectrometric, PicoTag, and hexose analyses of the tryptic 1-16 N-terminal peptides further indicated that the post-translational glycosylation of plasma membrane rhodopsin is identical to that of disk membrane rhodopsin. HPLC analysis of soluble peptides obtained from cyanogen bromide and tryptic digestion of immunoaffinity purified rhodopsin also indicated that no significant differences exist between disk and plasma membrane rhodopsin. Light-induced phosphorylation of rhodopsin in disk and plasma membranes were also compared using in vitro phosphorylation assays. Plasma membrane rhodopsin was found to undergo light-dependent, rhodopsin kinase catalyzed phosphorylation to the same extent as disk membrane rhodopsin. These results indicate that the bulk rhodopsin in rod outer segment plasma membranes appears to be identical to rhodopsin in disk membranes in regard to primary structure, post-translational glycosylation and light-dependent phosphorylation. On this basis, it is unlikely that the sorting of rhodopsin between disk and plasma membranes occurs by a mechanism based on differences in structural properties of rhodopsin.

Extracellular glucose dependence of rhodopsin regeneration in the excised mouse eye

To study the process of rhodopsin regeneration a superfused excised whole eye preparation of the albino mouse was developed. With this preparation, complete regeneration could be observed after each of the first two illuminations (bleaching 15-20%), and incomplete regeneration after a third illumination. Regeneration was minimal at extracellular glucose concentrations of 0 or 1 mM with improved regenerations at higher concentrations. Maximum regenerations were observed at glucose concentrations of 4-10 mM. First-bleach regenerations were as follows: 0 mM glucose, 20%; 1 mM, 8%; 2 mM, 45%; 3 mM, 82%; 4 mM, 115%; 5.1 mM, 121%; 7 mM, 120%; and 10 mM, 126%. The effects of reduced glucose were reversible. After an initial bleach with 0 or 1 mM extracellular glucose that exhibited minimal regeneration, the re-addition of glucose (5.1 mM) restored the ability of the eye to regenerate rhodopsin following a second bleach, but only to the level prior to that bleach. Mitochondrial substrates fumarate (10 mM) or pyruvate (10 mM) partly substituted for glucose, exhibiting first-bleach regenerations of 56 and 85%, respectively.

Glycolysis and glucose uptake in intact outer segments isolated from bovine retinal rods

Glucose transport across the plasma membrane of isolated bovine rod outer segments (ROS) was measured by uptake of 14C-labeled 3-O-methylglucose and 2-deoxyglucose and was inferred from deenergization of ROS with 2-deoxyglucose. Glucose transport was mediated by a facilitated diffusion glucose transporter that equilibrated external and internal free hexose concentrations. Glucose transport in ROS displayed two components as judged from kinetic analysis of hexose equilibration and as judged from inhibition by cytochalasin B and phloretin. Transport under exchange conditions was considerably faster as compared with net hexose uptake, similar to that observed for the erythrocyte glucose transporter. Sensitivity to cytochalasin B and affinity to 3-O-methylglucose were similar to those observed for the hepatocyte glucose transporter. The cytochalasin-insensitive component appears unique to ROS and did not reflect leakage transport as judged from a comparison with L-glucose uptake. Glucose transport feeds glycolysis localized to ROS. We suggest that a major role for glycolysis in ROS is phosphorylation of GDP to GTP via pyruvate kinase and PEP, while phosphorylation of ADP to ATP can use the creatine kinase/phosphocreatine pathway as well.

The cyclic nucleotide-gated channels of vertebrate photoreceptors and olfactory epithelium

Cation channels that are directly gated by guanosine 3', 5'-cyclic monophosphate (cGMP) control the flow of ions across the surface membrane of vertebrate rod and cone photoreceptor cells. A similar channel, gated by adenosine 3',5'-cyclic monophosphate (cAMP), exists in vertebrate olfactory sensory neurons. The channel polypeptide of rod photoreceptors has been identified and the amino acid sequence of the channel polypeptide in rod and olfactory cells has been determined by cloning cDNA. Although the cyclic nucleotide-gated channels functionally belong to the class of ligand-gated channels, they share some structural features with voltage-gated channels.